Priority is claimed to Patent Application Number 2001-11472 filed in Rep. of Korea on Mar. 6, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method for manufacturing a flat fluorescent lamp, and more particularly, to a method for manufacturing a flat fluorescent lamp in which formation of a fluorescent layer is more efficient.
2. Description of the Related Art
Flat panel displays are roughly divided into a light emitting type such as cathode ray tubes (CRTs), electroluminescent (EL) devices, plasma display panels (PDPs), and a light receiving type such as liquid crystal displays (LCDs). LCDs need an additional light source, for example a backlight, to display images because liquid crystal itself cannot emit light.
A typical back lighting arrangement for LCDs employs cold cathode fluorescent lamps (CCFLs) or flat fluorescent lamps with a fluorescent layer deposited on a plate. The back lighting arrangement of CCFLs is also divided into an edge-light type in which CCFLs are stacked at sides of the lamp and light from the CCFLs are guided to entire lamp surface by a light guiding plate, and a direct-light type in which CCFLs are placed directly under the front plate of the back light.
A CCFL is constructed by connecting a plurality of fluorescent lamps in parallel on a plate and is operated with the application of alternating current (AC) voltage. The plurality of fluorescent lamps need separate connections to a power source for selective emission to display a particular image. Edge-light type CCFLs are incompatible with large-sized panels because light is emitted from only one edge therein. For these reasons, the need for LCD back lighting with flat fluorescent lamps is increasing.
FIG. 1 is a perspective view showing the structure of a conventional flat fluorescent lamp. Referring to FIG. 1, the conventional flat fluorescent lamp includes a front plate 11 and a back plate 12 arranged to face the front plate 11. A fluorescent layer 13 is deposited on the bottom of the front plate 11. A series of discharging electrodes are arranged on the top of the back plate 12 with a predetermined separation gap, and a dielectric layer 15 is formed covering the discharging electrodes 14. A reflective layer 16 is formed on the dielectric layer 15, and another fluorescent layer 18 is formed on the reflective layer 16. In addition, the conventional flat fluorescent lamp includes spacers (not shown) providing a gap between the front plate 11 and the back plate 12 and a discharge gas injected into a sealed space between the front plate 12 and the back plate 12.
In the conventional flat fluorescent lamp having the configuration described above, as power is applied to the discharging electrodes 14, the LCD screen is irradiated with light generated by exciting the fluorescent layers 13 and 18 with ultraviolet (UV) light generated by discharge gas. In particular, the fluorescent layer 18 formed on the back plate 12 of the flat fluorescent lamp not only serves to convert the UV light generated through gas discharge into visible light but also prevents luminance loss by blocking visible light transmission through the bottom of the back plate 12. The fluorescent layer 18 formed on the back plate 12 of the flat fluorescent lamp should be as thick as 100-120 xcexcm.
A screen printing method is widely used as a fluorescent layer formation method for the conventional flat fluorescent lamp. A single screen printing produces a layer having a thickness of 7-10 xcexcm that is not thick enough as a fluorescent layer for the flat fluorescent lamp. As another drawback of the screen printing method, application of the screen printing method is limited to only a uniform print surface or a particular pattern. In general, formation of the florescent layer 18 on the back substrate 12 follows spacer formation. Therefore, there is a difficulty in applying the screen printing method to form a fluorescent layer covering protruding spacers on a back plate.
In the screen printing method, phosphor to be deposited on a plate is prepared as paste mixed with a large amount of an organic binder. Here, the large amount of the organic binder may cause a problem of outgassing because it remains after packaging of the display device.
As another approach to forming a fluorescent layer, U.S. Pat. No. 5,509,841 discloses flowing of slurry from one end of a plate with partitioned paths. However, this method is not suitable to form a uniform, thick fluorescent layer on a larger plate.
To solve the above-described problems, it is an object of the present invention to provide a method for manufacturing a flat fluorescent lamp in which a fluorescent layer can be formed to a sufficient thickness with easy thickness control, and phosphor can be deposited with less organic binder.
To achieve the object of the present invention, there is provided a method for manufacturing a flat fluorescent lamp, the method comprising: preparing a transparent front plate; preparing a transparent back plate facing the front plate, the back plate having a plurality of discharging electrodes thereon and a dielectric layer covering the discharging electrodes; forming a plurality of spacers between the front plate and the back plate to keep a separation gap therebetween; and forming a fluorescent layer by spraying phosphor slurry on one surface of at least one of the front plate and the back plate.
It is preferable that the phosphor slurry contains 40-60% by weight phosphor based on the total weight of the phosphor slurry. If the content of the phosphor is less than 40% by weight, a longer spraying time is required to obtain a fluorescent layer having a target thickness, and the phosphor slurry has a low viscosity, thereby causing a slurry flow off of the plate. In other words, efficient formation of a thick fluorescent layer is difficult. In contrast, if the content of the phosphor exceeds 60% by weight, it is not easy to supply the phosphor slurry through a pipeline into a spray equipment due to increased viscosity of the phosphor slurry.
It is preferable that the phosphor slurry contains 1-5% by weight an organic binder based on the total weight of the phosphor slurry. If the content of the organic binder is less than 1% by weight, adhesion of the phosphor to the plate is reduced. If the content of the organic binder exceeds 5% by weight, a problem of outgassing would occur as a result of organic binder remaining after device packaging.
Preferably, the organic binder comprises at least one selected from the group consisting of ethylcellulose and nitrocellulose. Preferably, the solvent comprises at least one selected from the group consisting of ethanol, terpineol, and 2-(2-butoxyethoxy)ethylacetate (BCA). Preferably, a mixture of ethanol and terpineol used as the solvent has a mixing ratio of 3:1 by weight.